It’s thunder-storming here in the northeastern state of Massachusetts. And I thought I’d give my best effort to give a general gist as to why the lightning and thundering occurs.
First, as we were taught in grade school, water evaporates and condenses into clouds. The air and water molecules inside these clouds however are not at rest. They are constantly moving, hitting, and rubbing against each other. As a result the molecules gain or lose charges from each other: some molecules become negative and some molecules become positive. The positive molecules rise to the top of the cloud while the negative molecules sink to the bottom of the cloud. Eventually, the imbalance of charges has to give, and the cloud discharges in the form of lightning to rebalance the charges. In most thunderclouds, the lightning occurs inside the cloud, an intracloud discharge. We see these as bright flashes in the sky. However, there are occasions when the lightning is strong enough that it breaks through the insulation, that is air, and discharges on the ground, earth, a cloud to ground discharge. We see these as distant bolts in the sky. (In America, we like to say we ground our electrical circuits, but in England they say they earth their electrical circuits. Same thing, tomato, tomato. Haha, it doesn’t quite work in writing.)
So that explains lightning, what about thunder? Well, as lightning is produced, vibrations are also produced. And any form of vibration is a sound, whether we can hear it or not. So for intracloud discharges, the sound we hear is a soft rumbling, since it all occurs inside the cloud. These rumbles are typical of springtime thunderstorms and some find them calming and relaxing. For cloud to ground discharges, the vibrations are louder and make a crashing sound. And at any one of these cloud to ground discharge, the lightning bolt also has branches, and these branches cause the additional crackling sound we hear before the main loud crash. So there you have it, if you hear rumbling, the lightning is most likely inside the cloud. If you hear a loud crash, then find safety because those are most likely cloud to groud discharges and if one just so happens to strike you, you could die. And if you hear crackling, those are the branches of the main cloud to ground discharge, and I would probably also find safety.
A meteorologist will probably give you a much better explanation . . .
Here’s a short clip I found on Discovery. Check it out, it shows some great looking thunderclouds.